Markets for electronic contents such as, for example, electronic books, digital videos, digital audios, games, etc. have experienced phenomenal growth in the last few years. Electronic book purchases, via an online bookstore, are growing in proportion relative to the purchase of physical books. The same is true regarding the purchase of music files via an online music shop.
To promote sales of the electronic contents, content providers provide previews for the electronic contents. For example, the content providers allow potential purchasers to read first few pages of an electronic book, or to listen first few seconds of a digital song.